For me, other grown people can talk about whatever they wish to talk about! If your fiance is talking about her sexual preferences, that's her business; if she is talking about your sexual preferences, that is not her business.

If you do not like what your fiance talks to someone else about (presuming she shares it with you) or her behavior, then perhaps she isn't the woman you "can" get yourself to marry. Presumably if she talked about subjects you did not like or which you found in poor taste or were boring to you, etc. you wouldn't have been attracted to her and dated her/asked her to marry you. But her expression of her self, her likes and dislikes, whether they be sexual or anything else, is who she "is". That does not make her (or him) "bad" or cheaters or anything else. I would consider, that she is sharing all of her information with you, wanting you to meet her new friend, wanting you to like him too, etc. This tells you about her.

My husband and I had a couple who we were friends with and the guy was a working man and liked dirty jokes. My husband and I don't particularly care for them but we didn't take his telling dirty jokes personally! That was about him and, otherwise, we enjoyed being with him.

Your fiance has a lot to her. Some of it you probably don't know yet, because you haven't been married to her 20 years :-) That's part of what marriage is all about, getting to know the other person.

But she is your equal and can be interested in whomever she likes, and be friends with whomever she likes. She'd like you to be friends with this person too but it does not sound like he's your kind of guy (to put it mildly :-) But their friendship is not your problem. Knowing she likes this guy and likes occasionally talking about sex, etc. and whether you want to be with someone who likes those things, is your problem. She doesn't like/love you less, because she likes being friends with this guy. I wouldn't necessarily call you "jealous" (unless you do not trust your fiance and/or are insecure in your own sense of self that you are afraid this new friend could win your fiance away from you).

But I'm glad your told your fiance that her talking to the guy made you uncomfortable. It sounds like you both have a fairly good start on good communications. I would continue talking with her about this, tell her that you are afraid for the relationship because of your discomfort. I think if she were not so forthcoming with information about her talks with him and showing you everything and wanting you to meet him and become friends with him that then you would be in a different, more worrisome position of wondering if you could trust her but, from what you've said, I would trust her because of her transparency. However, to me, it sounds like you really don't like this guy and may be afraid that some how he could win her away from you? Or someone else talking about sex "embarrasses"/"worries" you? I don't think "all guys" talk about sex (like your fiance seems to think) but I think that may be coming from her experience (which makes me think she's not very old/experienced/hasn't been around that many men in different social situations) and maybe you can talk to her about that image she has of guys and how, if it is her experience, it makes you unhappy (since it's not true and there's lots more to guys than just sex)?